Method and computer for determining a setting for correct operation of an internal combustion engine

ABSTRACT

This involves setting the operation of an internal combustion engine as a function of the air pressure in at least one engine air inlet manifold ( 5, 7 ), in which a campaign of bench tests is carried out on the engine in order to establish a law governing the evolution of the air flow rate MAF of the air entering a cylinder ( 1 ) of this engine, as a function of the admission pressure MAP taken from the inlet manifold, this being done at various engine speeds and then, on an operating engine, the air pressure is measured in said engine inlet manifold, a corresponding air pressure MAP value is supplied to a computer ( 27 ) that is also supplied the engine speed corresponding to it, and, from this, said air flow rate is deduced as a function of the established evolution law.  
     According to the invention, for a series of engine speeds N in succession, and up to a certain pressure MAP value, said evolution law is established by modeling it using a corresponding series of refined straight lines  
       y=ax−b , with:  (a)  
     y=MAF  
     x=MAP  
     a=a 1 ×N (a 1  is a constant)  
     b=a 2 ×N (a 2  is a constant)  
     and, from a pressure MAP threshold onward, a corrected model is applied to take account of a difference between the actual value of the flow rate MAF and its value according to the model (a), beyond the threshold, to do that replacing b with b′, with:  
       b′=f ( N, MAP   2   , MAP   3 )+ a   3   ×N    
      so that the evolution in the flow rate MAF becomes a function not only of the engine speed N but also of the evolution to the second and third power of the pressure MAP.

[0001] The invention relates to a method for determining the amount ofair entering a cylinder of an internal combustion engine or series ofengines as a function of the air pressure in at least one engine inletmanifold.

[0002] One objective is to determine the air mass flow rate of air letinto the cylinder to which the aforesaid inlet manifold is connected, inorder in particular to define the correct air/fuel mixture, at the sametime minimizing emissions of polluting gases.

[0003] In this context, one problem that the invention addresses isconcerned with obtaining a more appropriate air flow rate to thecylinder(s), using for that a simple, effective but functional method.

[0004] To do this, it is already known practice to proceed as follows:

[0005] a campaign of bench tests is carried out on the engine or on anengine of the series in order to establish a law governing the evolutionof the air mass flow rate MAF entering a cylinder of the engineconnected to said inlet manifold, as a function of the admissionpressure MAP in this inlet manifold, this being done for various enginespeeds, then,

[0006] on an operating series engine:

[0007] the air pressure in said engine inlet manifold is measured,

[0008] a corresponding air pressure MAP value is supplied to a computerwhich is also supplied with the engine speed corresponding to it,

[0009] and, from this, said air mass flow rate is deduced as a functionof the established evolution law.

[0010] However, it has been found that, at least for some types ofengine, there is a discrepancy between the air flow rate value suppliedby the aforesaid computer as a function of said evolution law and theactual value of this flow rate, at least for certain values of theadmission pressure (pressure MAP) in the inlet manifold.

[0011] It will now be noted that this discrepancy occurs at least on“small engines”, namely gasoline engines, in which the volumes of theinlet manifold(s) are small, and in particular smaller than the cylindercapacity of the engine. It has even been found that a sector to whichthe solution of the invention more particularly applies is that in whichthe following relationship is satisfied:$\frac{V_{c} \times N_{c}}{C} < 3.5$

[0012] with V_(c): volume of the inlet manifold or of the manifold“plenum” where the pressure MAP measurements are taken,

[0013] N_(c): number of engine cylinders

[0014] C: cylinder capacity of the engine.

[0015] It will be noted for all practical purposes that the term“plenum” of an inlet manifold is applied to a chamber interposed on thismanifold between the member that regulates the admission air flow rate(typically the butterfly valve) and the inlet valve of the cylinder towhich said manifold is connected.

[0016] Particularly with this type of engine (although other engines maybe affected), a significant feature of the invention thereforeanticipates, in order to provide a solution to the aforesaiddiscrepancies, that, at least for certain values of the pressure MAP,the computer will be supplied, by way of evolution law, with a correctedmodel. However, given that test campaigns have demonstrated that atleast in many cases there is a pressure MAP threshold beyond which theair flow rate MAF no longer evolves as a substantially linear functionof the pressure MAP, for various engine rotational speeds N, this beingparticularly true of engines of small cylinder capacity, the inventionproposes, in order for the onboard computer to be able to supply theconditions of functional stoechiometric air/fuel mixture, that:

[0017] for a series of engine speeds N in succession, and up to acertain pressure MAP value, said evolution law is established bymodeling it using corresponding series of refined straight lines

y=ax−b, with:  (a)

[0018] y=MAF

[0019] x=MAP

[0020] a=a₁×N (a₁ is a constant)

[0021] b=a₂×N (a₂ is a constant)

[0022] and, from an pressure MAP threshold onward, a corrected model isapplied to take account of a difference between the actual value of theflow rate MAF and its value according to the model (a), beyond thethreshold, to do that replacing b with b′, with:

b′=f(N, MAP ² , MAP ³)+a ₃ ×N

[0023]  so that the evolution in the flow rate MAF becomes a functionnot only of the engine speed N but also of the evolution to the secondand third power of the pressure MAP.

[0024] In addition, for a first category of engines, it is evenadvisable, still in order to take account of the aforesaiddiscrepancies, that the procedure be as follows:

[0025] said law governing the evolution of the air flow rate as afunction of the pressure in the inlet manifold is established byconsidering that:

MAF=S ₁ ×MAP−Ofs(N)

[0026] with MAF: air flow rate of the air entering the cylinder,

[0027] S₁: constant,

[0028] N: engine speed,

[0029] MAP: air pressure in the inlet manifold connected to the cylinderin question,

[0030] Ofs (N): value, at a given engine speed N, of the flow rate MAFat zero pressure MAP,

[0031] and, at least for said certain pressure MAP values, a correctionis introduced into the value of the flow rate MAF using the correctedmodel so as to take account of a difference there is between the actualvalue of the flow rate MAF, at a determined value of the pressure MAPand a given engine speed N, and the one supplied by said establishedevolution law, this correction being determined from measurements madeduring the bench test.

[0032] Because, on certain engines tested, the evolution in the air flowrate MAF as a function of the admission air pressure MAP, at variousengine speeds N, depart markedly from a straight line under certainengine operating conditions, it will be noted that in actual fact it ispossible to envisage applying said corrected model to all the pressureMAP values.

[0033] In order to obtain the most precise value of the air flow rateMAF, it is, for all that, advised in the invention to supply thecomputer, at least for certain pressure MAP values, by way of acorrected model, that established according to the law:

MAF[(S ₀ ×MAP ³)−(S ₁ ×MAP ²)+(S ₃ ×MAP ²)]×N−Ofs(N), with:

[0034] S₀, S₁, S₃: constants,

[0035] Ofs (N): value of the air flow rate MAF, at zero pressure MAP andat the engine speed N.

[0036] One problem with which the invention is also faced is that of thebest way of taking account, if necessary, of the pressure MAP thresholdbeyond which the discrepancy in the evolution law MAF=f(MAP) isconsidered to degrade the results.

[0037] The solution proposed by the invention is to take account of anaveraged filtered pressure MAP. Indeed it is found that the mean of theair pressure MAP is a continuous function of the position of the memberthat regulates the air flow rate in the inlet manifold (typically theposition of the butterfly valve) and of the engine rotational speed.

[0038] In fact, it is even advised to define the admission air pressurevalue (threshold) beyond which the corrected model applies, taking thefollowing into consideration:

[0039] said filtered averaged pressure values,

[0040] and instantaneous admission air pressure values measured more orless upon opening of the inlet valve inserted between the cylinder inquestion and the inlet manifold in question.

[0041] In series engines in operation, the filtered averagedmeasurements of pressure MAP will be supplied to the computer forreal-time air/fuel metering. It will be noted that, particularly in thecase of the aforesaid “small engines”, the sampling will be performedasynchronously with regards to the rotational cycle of the engine.

[0042] Apart from the foregoing, the invention also relates to anelectronic computer installed in an internal combustion engine todetermine a setting for correct operation of this engine, one feature ofthe computer being, according to the invention, that in this computer ismodeled the law governing the evolution of the air flow rate MAF as afunction of the admission pressure MAP, as follows:

[0043] for a series of engine speeds N in succession, and up to acertain pressure MAP value, there is established in the computer the lawgoverning the evolution of the air flow rate MAF entering a cylinder (1)of the engine connected to an inlet manifold (5, 7) through which atleast air passes, as a function of the air pressure MAP taken from saidinlet manifold, by modeling it with a corresponding series of refinedstraight lines

y=ax−b, with:  (a)

[0044] y=MAF

[0045] x=MAP

[0046] a=a₁×N (a₁ is a constant)

[0047] b=a₂×N (a₂ is a constant)

[0048] and, from a pressure MAP threshold onward, a corrected model isapplied to take account of a difference between the actual value of theflow rate MAF and its value according to the model (a), beyond thethreshold, to do that replacing b with b′, with:

b′=f(N, MAP ² , MAP ³)+a ₃ ×N

[0049]  so that the evolution in the flow rate MAF becomes a functionnot only of the engine speed N but also of the evolution to the secondand third power of the pressure MAP.

[0050] A more detailed description of the invention will now be givenwith reference to the appended drawings in which:

[0051]FIG. 1 shows, for three different engine speeds, an evolution inflow rate MAF as a function of an admission air pressure MAP, this beingfor an indirect-injection gasoline engine with a cylinder capacity oftwo liters;

[0052]FIG. 2 shows three curves showing, for three different enginespeeds, an evolution in flow rate MAF as a function of pressure MAP,this being for a 125-cc four-stroke gasoline engine;

[0053]FIG. 3 schematically shows a cylinder of a gasoline engine with,in particular, an inlet manifold, a cylinder and an exhaust manifold.

[0054] Typically, it was known at the date of the invention that, for anumber of internal combustion engines (particularly indirect-injectiongasoline engines) and under standard atmospheric pressure andtemperature conditions in the engine cylinder(s), the air flow rate MAFof air let into this cylinder may be modeled using the followingformula:

MAF=S ₁ ×N×MAP−Ofs(N), with  (1)

[0055] MAF: flow rate of air entering the cylinder at a steady enginespeed

[0056] S₁: constant

[0057] N: engine speed

[0058] MAP: pressure in the (one of the) inlet manifold(s) connected tothe cylinder,

[0059] Ofs(N): non-zero constant corresponding to the value of the airflow rate MAF at the zero value of pressure MAP and at the engine speedN.

[0060] It has been possible to draw a series of curves as illustrated inFIG. 1 from such an evolution law.

[0061] The three curves illustrated each correspond to a differentengine speed from the other two.

[0062] It can be seen that, in this case, at the right-hand end of eachof the curves, for high air pressures MAP (of the order of 1000millibar), each curve begins to diverge from the refined straight linethat represents it over the remainder of the range of pressure MAPvalues.

[0063] In point of fact, it was found that the discrepancy could be fargreater and become detrimental to correct operation of the engine,particularly by disrupting the air/fuel mixture metering

[0064] As already stated, this was found to be the case in particular on“small cylinder capacity” engines where $\frac{V_{c}}{C} < 1.$

[0065] On this type of engine in particular (which are typicallyindirect-injection gasoline engines), it was found that, because of thesmall volume of the inlet manifolds and/or the small cylinder capacity,the pressure MAP values are clipped to atmospheric pressure when theinlet valve closes.

[0066] Furthermore, FIG. 2 depicts, for a 125-cc indirect-injectionsingle-cylinder four-stroke gasoline engine, a series of curves showingthe evolution of the air flow rate MAF as a function of the pressureMAP, and it is indeed found that, in the right-hand part of the curves,there is a discrepancy that is even more pronounced than that one inFIG. 1, this discrepancy occurring at pressures MAP that differaccording to the engine speed.

[0067] This is why in the invention a choice has been made to supply thecomputer with a modeled law governing the evolution of the air flow rateMAF, taking account of the evolution to the second and/or third power ofthe pressure MAP at various engine speeds N.

[0068]FIG. 3 shows the way in which this effect is implemented.

[0069] In this figure, reference 1 schematically denotes a cylinder 1 ofthe aforesaid 125-cc engine, with an exhaust manifold 3 and an inletmanifold 5 for letting in the fluid needed for engine operation.

[0070] The inlet manifold 5 comprises a first part 5 a and a second part5 b between which parts an “inlet plenum” 7 is interposed, through whichplenum only air flows and which constitutes a chamber of larger crosssection than the manifold portions 5 a, 5 b. The portion 5 b isconnected to the combustion chamber (or explosion chamber) 9 through theport 11 in which the inlet valve 13 is located. An exhaust valve 15controls the passage between the chamber 9 and the exhaust manifold 3.

[0071]17 denotes the piston that moves in the cylinder 1.

[0072] The fuel inlet has not been depicted.

[0073]19 denotes the member that regulates the admission air flow rate(typically a pivot-mounted butterfly valve) arranged inside the manifoldportion 5 a upstream of the inlet plenum 7 (which could be omitted).

[0074] Further upstream still, the inlet manifold portion 5 a isconnected to the air filter depicted diagrammatically at 21.

[0075] At 23, an air pressure probe will allow pressure measurements tobe taken at the plenum 7 on an operating series engine, to provide thecorrect air/fuel mixture “in real time” on the basis of, here, knowledgeof the air flow rate MAF.

[0076] This will involve sampled pressure measurements, and this willpreferably be done asynchronously with regards to the engine rotationcycle. Electronic filtering will preferably be used in conjunction withthese measurements, so as to obtain averaged digital data.

[0077] The rate of acquisition of the measurements could, for example,be every four to five milliseconds.

[0078] Still in FIG. 3, there is meanwhile a diagrammatic representationat 25 of an analog/digital converter connected to the sensor 23 and, at27, the digital filter that receives the converted data beforetransmitting them, once filtered, to a digital computer 29 designed inparticular to determine the most appropriate air flow rate MAF andtherefore to define the proper stoechiometric fuel/air ratio.

[0079] It will be noted that if asynchronous pressure measurements aretaken, it will be possible to use a less sophisticated computer.

[0080] Thus, it will advantageously be air pressure MAP values averagedby electronic filtering that will be supplied to the computer in orderto obtain the air flow rate MAF values.

[0081] For that, a low-pass filter with a time constant given as afunction of at least one of the following parameters will advantageouslybe chosen:

[0082] variation in the rotational speed of the engine;

[0083] gradient of opening or closure of the regulating member 19;

[0084] engine acceleration.

[0085] Thus, if the pressure MAP value supplied at the output from the“sliding” digital filtering, that is to say filtering making it possibleto obtain, at a filtering step of rank n, an averaged value that is afunction of the results of step n−1 and replacing it, reaches or exceedsa determined value, it will then be the evolution law (2) below thatwill be taken into consideration in the computer to provide the flowrate MAF.

[0086] The model for this evolution law entered into the computer willpreferably be as follows:

MAF=[(S ₀ ×MAP ³)−(S ₁ ×MAP ²)+(S ₃ ×MAP)]×N−Ofs(N), with:  (2)

[0087] S₀, S₁, S₃: constants

[0088] Ofs (N): value of the air flow rate MAF at zero pressure MAP andat the engine speed N.

[0089] As has already been understood, this evolution law that takesaccount of the second and third power of the admission air pressure MAPwas established after a campaign of bench tests on the same engine, ormore generally an engine of the same series.

[0090] If a pressure MAP threshold, on the basis of which the computeris made to establish the air flow rate MAF to be employed, using notformula (1) but formula (2), needs to be determined, then this thresholdwill advantageously depend on an averaged value supplied at output ofthe digital filter 27.

[0091] In this respect, it must be noted that the pressure MAP is acontinuous function that evolves with the position (openness) of theregulating member 19 and the rotational speed N of the engine.

[0092] To best take account of the evolution of this pressure in anoperating engine, it is advised meanwhile that the aforementionedthreshold value (defined of course during the initial bench testcampaign) take account of:

[0093] said filtered averaged pressure values,

[0094] and instantaneous admission pressure values measured more or lessupon opening of the inlet valve inserted between the cylinder inquestion and the inlet manifold in question.

[0095] It will also be noted that taking account in this way of theevolution to the second and/or third power of the filtered pressure MAP,for various engine speeds N, makes it possible to provide engine settingdata using a relatively unsophisticated computer, this being all themore true if this computer is made to take pressure MAP measurements inthe plenum in the form of measurements sampled asynchronously withregards to the engine operating cycles.

1. A method for determining a setting for correct operation of aninternal combustion engine or series of engines as a function of the airpressure in at least one inlet manifold (5, 7) of the engine, in whichmethod: a campaign of bench tests is carried out on the engine or on anengine of the series in order to establish a law governing the evolutionof the air flow rate MAF entering a cylinder (1) of the engine connectedto said inlet manifold, as a function of the admission pressure MAPtaken from this inlet manifold, this being done for various enginespeeds, then, on an operating series engine: the air pressure in saidengine inlet manifold (5, 7) is measured, a corresponding air pressureMAP value is supplied to a computer (27) which is also supplied with theengine speed N corresponding to it, and, from this, said air flow rateis deduced as a function of the established evolution law, characterizedin that: for a series of engine speeds N in succession, and up to acertain pressure MAP value, said evolution law is established bymodeling it using corresponding series of refined straight lines y=ax−b,with:  (a) y=MAF x=MAP a=a₁×N (a₁ is a constant) b=a₂×N (a₂ is aconstant) and, from a pressure MAP threshold onward, a corrected modelis applied to take account of a difference between the actual value ofthe flow rate MAF and its value according to the model (a), beyond thethreshold, to do that replacing b with b′, with: b′=f(N, MAP ² , MAP³)+a ₃ ×N  so that the evolution in the flow rate MAF becomes a functionnot only of the engine speed N but also of the evolution to the secondand third power of the pressure MAP.
 2. The method as claimed in claim1, characterized in that: said law governing the evolution of the airflow rate as a function of the pressure in the inlet manifold (5, 7) isestablished by considering that: MAF=S ₁ ×MAP−Ofs(N) with MAF: air flowrate of the air entering the cylinder, S₁: constant, N: engine speed,MAP: air pressure in the inlet manifold connected to the cylinder inquestion, Ofs (N): value, at a given engine speed N, of the flow rateMAF at zero pressure MAP, and, at least for said certain pressure MAPvalues, a correction is introduced into the value of the flow rate MAFusing the corrected model so as to take account of a difference there isbetween the actual value of the flow rate MAF, at a determined value ofthe pressure MAP and a given engine speed N, and that supplied by saidestablished evolution law, this correction being determined frommeasurements made during the bench test.
 3. The method as claimed ineither one of the preceding claims, characterized in that, at least forsaid certain pressure MAP values, the computer (27) is supplied, by wayof corrected model, with the model established according to the law:MAF=[(S ₀ ×MAP ³)−(S ₁ ×MAP ²)+(S ₃ ×MAP)]×N−Ofs(N), with: S₀, S₁, S₃:constants, Ofs (N): value of the air flow rate MAF, at zero pressure MAPand at the engine speed N.
 4. The method as claimed in any one of thepreceding claims, characterized in that the corrected modeling isapplied when $\frac{V_{c}}{C} < 1$

with V_(c): volume of the inlet manifold (5) or the plenum (7) of thismanifold, where the pressure MAP measurements are taken, C: cylindercapacity of the engine.
 5. The method as claimed in any one of thepreceding claims, characterized in that the corrected modeling isapplied when $\frac{V_{c} \times N_{c}}{C} < 3.5$

with V_(c): volume of the inlet manifold (5) or of the plenum (7) ofthis manifold, where the pressure MAP measurements are taken, N_(c):number of engine cylinders, C: cylinder capacity of the engine.
 6. Themethod as claimed in any one of the preceding claims, characterized inthat the computer is supplied a pressure MAP that has been measured inthe (one of the) inlet manifold(s) (5, 7) by a pressure sensor and thatis then passed through a digital filter (27) supplying an averagedpressure value.
 7. The method as claimed in claim 6, characterized inthat, in order to determine said at least certain pressure MAP values towhich the corrected model applies, an admission pressure value isdefined beyond which the corrected model applies, taking account of thefollowing: said filtered averaged pressure values, and instantaneousadmission air pressure values measured more or less upon opening of theinlet valve (13) inserted between the cylinder in question and the inletmanifold in question.
 8. A computer installed in an internal combustionengine to determine a setting for correct operation of this internalcombustion engine or series of engines, characterized in that: for aseries of engine speeds N in succession, and up to a certain pressureMAP value, there is established in the computer the law governing theevolution of the air flow rate MAF of the air entering a cylinder (1) ofthe engine connected to an inlet manifold (5, 7) through which at leastair passes, as a function of the air pressure MAP taken from said inletmanifold, by modeling it with a corresponding series of refined straightlines y=ax−b, with:  (a) y=MAF x=MAP a=a₁×N (a₁ is a constant) b=a₂×N(a₂ is a constant) and, from a pressure MAP threshold onward, acorrected model is applied to take account of a difference between theactual value of the flow rate MAF and its value according to the model(a), beyond the threshold, to do that replacing b with b′, with: b′=f(N,MAP ² , MAP ³)+a ₃ ×N  so that the evolution in the flow rate MAFbecomes a function not only of the engine speed N but also of theevolution to the second and third power of the pressure MAP.